Recently, a great deal of attention has been directed to environmental concerns throughout the world, and many countries have strictly strengthened regulations on toxic substances associated with waste disposal of used electric/electronic products. In general, an epoxy resin composition uses a brominated bifunctional epoxy resin and multifunctional epoxy resin as main components and additionally an amine curing agent and cure accelerator, and in order to meet the standard for flammability of materials, i.e., Underwriters Laboratory flammability classification 94V-0, the epoxy resin composition contains 15 to 20% by weight of bromine. However, halogen compounds including bromine provide excellent flame retardancy, but cause generation of toxic gases when they are burned and the probable production of carcinogenic substance dioxin. As such, phosphorus or nitrogen containing compounds or flame retardant inorganic materials have been recently studied as replacements for bromine-containing flame retardants.
Methods involving an addition of a phosphorus flame retardant as the non-halogen flame retardant are most widely used in the art. For example, Japanese Patent Publication Laid-open Nos. 2002-88139, 2002-161191 and 2002-161197 disclose a composition containing a phosphorus compound and a nitrogen compound as a flame retardant. In addition, U.S. Pat. No. 6,353,080 and Japanese Patent Publication Laid-open No. 10-195178 disclose a technique involving reaction of an epoxy resin using a reactive phosphoric ester-based flame retardant. However, when a copper-clad laminate is prepared using the phosphorus-based flame retardant which was adopted in the above-mentioned patents, it was found that an unreacted phosphorus compound is very highly likely to lower lead heat resistance and copper peel strength. Germany Patent DE 43 08 184 solves such problems via formation of a bifunctional or multifunctional epoxy resin which was previously reacted with the phosphorus-based flame retardant, but disadvantageously suffers from relatively poor humidity resistance and heat resistance as compared to conventional products.
As such, conventional non-halogen epoxy resin compositions do not provide well-balanced various physical properties which are to be required in copper-clad laminates.